Combining light sources of different spectra permit lighting devices to emit a light spectrum of almost any desired energy content. For example, red light can be combined with unsaturated green light to yield a light spectrum that renders colors similar to daylight or similar to incandescence depending on the amount of accompanying blue light. Using red, green, and blue light sources, colors from such sources can be combined in any proportion to yield any aggregate color within the gamut of colors.
Color is the visual effect that is caused by the spectral composition of the light emitted, transmitted, or reflected by objects. Color derives from the spectrum of light (i.e., distribution of light power versus wavelength) interacting in the eye with the spectral sensitivities of light receptors, including cones that are differently receptive to red, green, and blue light. Color categories and physical specifications of color are also associated with objects, materials, light sources, and the like based on their physical properties such as light absorption, reflection, and emission spectra.
Color is subjective, since it is generated within the visual cortex. Unlike the sensations of taste, smell, or feeling, color is not a characteristic of objects, but instead of the light that enters our eyes from the objects. Objects are visible or seen as colored only when light reaches our eyes after interaction with such objects. The same object may be seen in different colors when observed under varying lights. In the absence of light, all colors disappear.
Color reproduction may be performed according to additive or subtractive processes. One example of an additive color reproduction process is RGB color applied to a light-emitting color television or monitor. In such a context, images may be formed on an initially dark screen by illuminating pixels with light sources of basic colors (e.g., red, green, and blue), wherein any two basic colors in combination produce secondary colors (cyan, magenta, or yellow), and all three basic colors in combination produce white.
One example of a subtractive color reproduction process is ink printing on an initially white substrate. Since white is the presence of all color, the printing of colored ink onto an initially white substrate effectively “subtracts” color from the substrate. Typically, cyan (C), magenta (M), and yellow (Y) inks are used in color printing processes. Cyan can be thought of as minus-red, magenta as minus-green, and yellow as minus-blue. These inks are semi-transparent or translucent. Where two such inks overlap on a substrate (e.g., paper) due to sequential printing impressions, a primary color is perceived. For example, yellow (minus-blue) overprinted by magenta (minus-green) yields red. Hues are produced by overlapping and spacing the absorption spectra of the different inks on a reflective substrate. The ink density or thickness is typically constant but the area reflected by each ink is variable. White point is not affected by the ink. In regions where all three inks overlap, almost all incident light is absorbed or subtracted, yielding a near-black condition. Due to imperfect pigments and dyes, however, a combination of cyan, magenta, and yellow inks does not fully extinguish color of a light colored substrate, and typically yields a dark muddy color. To enhance the ability to produce black color, black ink is often added (referred to as “K,” which stands for “key” as a traditional word for a black printing plate). The resulting combination of four inks (cyan, magenta, yellow, and black) is commonly referred to as “CMYK.” When CMYK inks are used on a white substrate, there are effectively only eight spectra—namely black, white (substrate), cyan, yellow, magenta, blue (obtained by cyan+magenta), green (obtained by yellow+cyan), and red (obtained by yellow+cyan). Black and white may be regarded as neutral.
Most printed material uses Process Blue (PB15:3), Process Red (PR57-1), and Process Yellow (PY-12) inks. ISO Standard No. 2846-1-2006 entitled “Graphic technology—Colour and transparency of printing ink sets for four-colour printing” defines a method of testing inks to ensure acceptable color performance, and provides “typical” spectra for acceptable inks. A plot of reflected intensity versus wavelength for a white substrate as well as four standard CMYK inks derived from the above-identified ISO standard is provided in FIG. 1A. FIG. 1B includes the diagram of FIG. 1A with addition of a dashed oval at the intersection between the blue and yellow reflectance spectra. The presence of imperfect boundaries (including overlaps) between colored inks, such as within the dashed oval shown in FIG. 1B, causes reflected colors to be less vibrant and therefore muddled.
Additive and subtractive color reproduction processes differ in the manner they work, but ultimately achieve the same objective of providing reflection of three basic colors of red, green, and blue that are perceptible by the human eye under appropriate conditions. Because additive color spaces are defined by light and not colorants, devices employing additive color reproduction processes generally have a larger color gamut than devices employing subtractive color reproduction processes. As a result, vivid colors are more challenging to produce using a subtractive color reproduction process than using an additive color reproduction process.
Quality artificial lighting generally attempts to emulate the characteristics, including color rendering characteristics, of natural light. Natural light sources include daylight with a relatively high correlated color temperature (CCT) (e.g., ˜5000K), and incandescent lamps with a lower CCT (e.g., ˜2800K). A commonly accepted measure of color reproduction for light sources is Color Rendering Index (CRI) or average Color Rendering Index (CRI Ra). To calculate CRI, the color appearance of 14 reflective samples is simulated when illuminated by a reference radiator (illuminant) and the test source. The general or average color rendering index CRI Ra is a modified average utilizing the first eight indices, all of which are pastel colored with low to moderate chromatic saturation. (R9 is one of six saturated test colors not used in calculating CRI, with R9 embodying a large red content.) CRI and CRI Ra are used to determine how closely an artificial light source matches the color rendering of a natural light source at the same CCT. Daylight has a high CRI Ra (approximately 100), with incandescent bulbs also being relatively close (CRI Ra greater than 95), and fluorescent lighting being less accurate (with typical CRI Ra values of approximately 70-80).
CRI Ra (or CRI) alone is not a satisfactory measure of the benefit of a light source, since it confers little ability to predict color discrimination (i.e., to perceive subtle difference in hue) or color preference. There appears to be a natural human attraction to brighter color. Daylight provides a spectrum of light that allows the human eye to perceive bright and vivid colors, which allows objects to be distinguished even with subtle color shade differences. Accordingly, it is generally recognized that daylight and blackbody sources are superior to many artificial light sources for emphasizing and distinguishing color. The ability of human vision to differentiate color is different under CCT conditions providing the same CRI Ra. Such differentiation is proportional to the gamut of the illuminating light.
Gamut area of a light source can be calculated as the area enclosed within a polygon defined by the chromaticities in CIE 1976 u′v′ color space of the eight color chips used to calculate CRI Ra when illuminated by a test light source. Gamut area index (GAI) is a convenient way of characterizing in chromaticity space how saturated the illumination makes objects appear—with a larger GAI making object colors appear more saturated. GAI is a relative number whereby an imaginary equal-energy spectrum (wherein radiant power is equal at all wavelengths) is scored as 100. GAI for a test source is determined by comparing color space area of the light being tested to the color space area produced by the imaginary or theoretical equal-energy spectrum (EES) source. Unlike CRI Ra (or CRI), which has a maximum value of 100, GAI can exceed 100, meaning that some sources saturate colors more than an equal-energy source serves to saturate color.
It is found that typical blackbody-like light sources and typical daylight-like light sources have different gamut areas. Low CCT sources (e.g., incandescent emitters) have a GAI of approximately 50% (i.e., about half the gamut area of the EES source). Sources with higher CCT values have a larger GAI. For example, a very bluish light with a CCT of 10000K may have a GAI of 140%.
Another way of characterizing how saturated an illuminant makes objects appear is relative gamut area, or “Qg” (also referred to as “Color Quality Scale Qg” or “CQS Qg”), which is the area formed by (a*, b*) coordinates of the 15 test-color samples in CIELAB normalized by the gamut area of a reference illuminant at the same CCT and multiplied by 100. In a manner similar to GAI, Qg values can exceed 100; however, Qg values are scaled for consistency relative to CCT. Because of chromatic adaptation, and because CCT is selected to set the overall color tone of an environment as part of the lighting design process, variable-reference measures such as Qg may be especially relevant to applied lighting design. If the relative gamut is greater than that of the reference, and if illuminance is lower than that provided by daylight, then an increase in preference and discrimination might be expected relative to the reference at that same CCT. Conversely, if the relative gamut is smaller than that of the reference, then a decrease in preference and discrimination might be expected relative to the reference at the same CCT.
Significant resources are expended in illuminating printed material (e.g., including CMY or CMYK inks) on upright surfaces, such as posters, billboards, signs, packaging, point of purchase product displays, and the like. Billboards have traditionally been illuminated with fluorescent, metal halide, or phosphor converted white LEDs (such as disclosed in U.S. Patent Application Publication No. 2009/0077847A1) to provide high luminous efficacy at a relatively low cost. In certain contexts, it may be challenging to illuminate printed material with appropriate accuracy/naturalness of colors therein, to illuminate printed material with enhanced attractiveness, and/or to reduce the energy required to provide adequate illumination. The art continues to seek improved lighting devices and methods that address limitations of conventional lighting devices and methods.